This invention relates to a control system for a permanent-magnet motor drive, and more particularly to motor controllers utilizing the back EMF to obtain information for generating the inverter firing angle.
In some applications, a permanent-magnet (PM) motor is controlled by a line-commutated inverter driven by a dc source comprised of a phase delay rectifier (PDR). The PDR rectifies the 3-phase power from a utility line and delivers the resulting dc current to the inverter over a dc link choke. The dc link choke is designed to provide a smooth current source for the line-commutated inverter. The inverter provides the necessary current to the motor windings.
A motor speed loop is closed around the inverter control, and a change in required speed results in a change in the PDR firing angle. This in turn changes the dc link current, and hence the motor speed. It is possible to vary the current in the dc link from zero to a maximum by adjusting the firing angle of the PDR in response to a gain and loop compensated error signal, where the error is the difference between a speed command signal and a signal proportional to the speed of the motor generated by precision full-wave rectifier of the motor back EMF.
The back EMF is also utilized to generate the inverter control signals. Due to commutation overlap in the inverter, the back EMF is distorted. Consequently, the back EMF must first be filtered to remove distortions. This is generally accomplished with integrators, but in order to maintain a precise firing angle, expensive precision components must be used. A further problem is that the integrators provide much more gain to spurious low frequencies than to the actual operating frequency.
These spurious low-frequency fluctuations are caused by modulation between the PDR ripple and the back EMF frequency. At high motor speeds, these low-frequency fluctuations modulate the motor speed and are further reinforced by the integrators. Thus, in order to operate a line-commutated permanent-magnet motor at high speed, these low-frequency components must be filtered. A single high-pass filter, however, introduces undesirable phase shifts which result in poor motor power factors. The filter break (corner) frequency must be selected such that the phase shift does not cause excessive change in the power factor. It has been found that any single zero location that results in acceptable power factors will not adequately eliminate low-frequency noise at all speeds.
One solution to this filter problem is to provide a high-pass filter whose break frequency can be changed as a function of speed so that adequate low-frequency filtering and system power factors can be obtained at all motor speeds, as disclosed in copending application Ser. No. 291,132 filed Aug. 7, 1981 entitled "ADAPTIVE CONTROL SYSTEM FOR LINE-COMMUTATED INVERTERS." However, as the speed increases, the attenuation of low-frequency components increases. This changes the output amplitude of the filter. Since this output is compared to a constant voltage in generating firing angle signals for inverter firing angle control, any change in amplitude represents a change in the operating power factor of the system.
The dominant factor in the efficiency of a line-commutated inverter is its firing angle. This firing angle always represents a power loss. Consequently, what is required is a system for maintaining precise control of the inverter firing angle, thereby optimizing motor efficiency. From the foregoing, it follows that any such system must be immune to any amplitude fluctuations of the back EMF, or the integrated (filtered) back EMF. An object of this invention is to provide such a system without the use of expensive components in the back EMF integrator.